Recent resources of an oxide raw material tend to be reduced in grade. To improve the grade, the low-grade raw material has been concentrated to a fine or granular raw material by ore dressing.
As a method of producing metal or alloy from the fine or granular raw material, it is known that the fine or granular raw material is first preliminarily reduced, and is thereafter melt reduced to produce the metal or the alloy. A known system for preliminarily reducing the fine or granular raw material employs a rotary furnace or a fluidized bed furnace. The fluidized bed furnace has the following advantages in comparison with the rotary furnace.
(1) Temperature control is precisely carried out to thereby prevent formation of a mass of the fine or granular raw material due to local heating.
(2) A processing capacity in relation to an installation area is large.
Accordingly, it is expected that the fluidized bed furnace will be increasingly employed.
In general, the fine or granular raw material has a maximum particle size of 3 mm, and it also contains a powder having a particle size of 0.1 mm. The fine or granular raw material is fluidized by a reductive gas flowing at a velocity of 2-10 m/sec in a fluidized bed. In the case of producing a preliminarily reduced raw material at a rate of 100 t/day, a necessary size of the furnace is 2-6 m in inner diameter and 10 -30 m in height. In order to carry out a preliminary reducing reaction uniformly and efficiently, the fine or granular raw material and the reductive gas supplied need to be sufficiently dispensed and mixed with each other.
A known fluidized bed reducing furnace is disclosed in Japanese Patent Laid-open Publication No. 58217615, for example. This prior art furnace is a small-sized furnace having an inner diameter of about 1 m, and it includes a single raw material supply port and a substantially single reductive gas supply port. In this type furnace, it is difficult to sufficiently disperse and mix the raw material particles and the reductive ga with each other. Accordingly, in the development of a large-sized fluidized bed reducing furnace capable of producing a large amount of preliminarily reduced ore, there has been demanded a fluidized bed type preliminary reducing furnace of a type capable of sufficiently disperse and mix the raw material particles and the reductive gas with each other.